Recently, liquid crystal drivers have many functions, and their performance is getting higher. The number of outputs from liquid crystal drivers is therefore increasing. Accordingly, the pitch of carrier tapes for carrying liquid crystal drivers is rapidly getting finer. At present, COF (Chip On Film) is becoming a mainstream carrier tape, because the COF is a semiconductor carrier tape whose pitch can be finer than that of TCP (Tape Carrier Package).
A common method of assembling (manufacturing) a semiconductor device using the COF is as follows. First, metal wire is patterned by etching on a base film of polyimide. Then, the metal wire is tinned. In this way, a semiconductor carrier film is formed. After that, a semiconductor chip having a projecting electrode is bonded to the semiconductor carrier film by thermo compression. This bonding step is called “inner lead bonding (ILB)”. After the ILD, underfill resin (protective material) is filled into a space between the semiconductor chip and the semiconductor carrier film. The underfill resin is then cured. Assembly of the semiconductor device using the COF is completed by a final test.
The semiconductor carrier film using the base film mainly includes a film material produced by either a casting method or a metalizing method. In the casting method, the base film material is produced by applying varnish of polyimide onto copper foil having a thickness of 12 μm to 18 μm, and curing the varnish. In the metalizing method, the base film material is produced by forming a metal barrier layer on a polyimide base material by a spattering method, and coating the metal barrier layer with copper, thereby forming a copper film (layer) which is to be a wire. In order to attain a finer pitch, it is necessary that the copper film which is to be a wire is thin. Therefore, the metalizing method is more suitable than the casting method, because it is difficult to form a thin copper film by the casting method, whereas a thin film can be manufactured by the metalizing method by simply controlling the thickness of coating. The metalizing method is disclosed in Japanese Publication for Unexamined Patent Application, Tokukai 2002-252257 (publication date: Sep. 6, 2002), for example.
FIG. 8 illustrates a cross-sectional structure of a commonly used semiconductor carrier film formed by the metalizing method. In the metalizing method, a barrier layer of nickel-chrome alloy (chrome: 7% by weight, nickel: 93% by weight) having a thickness of approximately 50 Å to 100 Å (5 nm to 10 nm) is formed by spattering on a polyimide base material 110, which is to be a base. Then, in a commonly used method, spattering copper having a thickness of 1000 Å to 2000 Å is formed, followed by electric or electroless copper plating. In this way, a copper wire layer (thickness: approximately 8 μm) which is to be a wire pattern is formed. Next, in order to form a desired wire pattern on the film base material, photoresist is applied onto the copper wire layer, and the photoresist is cured. After masking is performed in a predetermined pattern, exposing, developing, copper etching, and photoresist peeling are performed. In this way, as shown in FIG. 8, (i) a barrier layer 102 and (ii) a wire layer 103 made of copper are formed. The barrier layer 102 and the wire layer 103 have a predetermined width. After the photoresist peeling, a tin coating or a tin-and-gold coating (not shown) is formed. Finally, a necessary part of the wire is coated with a solder resist 111. As a result, a semiconductor carrier film is produced.
However, in a semiconductor carrier film formed by the conventional metalizing method, if a distance between adjacent wires (terminals) having a potential difference is short, or if the potential difference between the adjacent terminals is large due to a high output, an insulating resistance between the adjacent terminals tends to deteriorate in the environment of high temperature and moisture, because migration occurs between the adjacent terminals having the potential difference. In particular, if the wire is coated with gold, the migration is more salient because a cyan-type solution, which is used as a plating liquid, slightly remains. As a result, there is a problem that neither a finer pitch nor a higher output can be attained.
FIG. 9 illustrates a mechanism of the occurrence of the migration.
FIG. 9 is a cross-sectional view of a conventional semiconductor carrier film. On a base film 110 of polyimide, a barrier layers 102 and wire layers 103a and 103b are formed. On surfaces of the barrier layers 102 and the wire layers 103a and 103b, tin coatings 104 are formed. On the tin coatings 104, gold coatings 105 are formed. The barrier layers 102 are made of nickel-chrome alloy including 7% by weight of chrome and 93% by weight of nickel. The thickness of the barrier layers 102 is 7 nm. There is a potential difference between the wire layer 103a and the wire layer 103b. The wire layer 103a has a positive potential, and the wire layer 103b has a negative potential or a GND potential.
When the conventional semiconductor carrier film is placed in the environment of high temperature and moisture, water droplets 106 adhere to the semiconductor carrier film. The water droplets 106 include an impurity such as chlorine. The droplets 106 penetrate through a porous part of the barrier layer 102 on the side of the wire layer 103a (the wire layer having the positive potential). As a result, a part of the barrier layer 102 elutes, as ions, into water, and moves toward the wire layer 103b (the wire layer having the negative potential or the GND potential). Through a barrier layer elution part 107, the copper forming the wire layer 103a (the copper which is a component of the wire layer 103a) is eroded, thereby creating an eroded part 109. The copper forming the wire layer 103a also elutes toward the wire layer 103b (the wire layer having the negative potential or the GND potential). In particular, the erosion of copper and the elution of (i) the copper forming the wire layer 103a and (ii) the component of the barrier layer 102 occur easily because the cyan-type solution usually used in forming the gold coating 105 remains without completely cleaned. As a result, the migration occurs due to a copper elution part 108 and the barrier layer elution part 107, thereby deteriorating the insulating resistance between the adjacent terminals.